The whole radio frequency spectrum has been split into regions according to ITU radio regulations.
The VLF region corresponds to frequencies between 3 kHz and 30 kHz (wavelengths between 100 and 10 km). Those frequencies are much lower than those used for AM broadcasts radio station (LW, MW, SW).

VLF waves are used for time signals and radio navigation beacons (such as the Russian hyperbolic radionavigation system RSDN-20. And, since they can penetrate water to a depth of tens of feets, they are used by militaries to communicate with submarines near the surface. Transmitters have a power of a few hundreds of kilo-Watts.

VLF frequency range is also home to natural electromagnetic emissions (called sferics, tweeks, whistlers...) emitted thousands of miles from the receiver. They can be turned into sound that we can hear. This is called "natural radio".

Choosing a transmitter for SID monitoring

Many military stations around the world use VLF for communications with submarines. You can found in this website a list of VLF transmitters that can be used for SID monitoring.

It is important to select a transmitter that is far enough to limit the ground wave field strength. The DCF77 web site (see DCF77 range) mentions a predominant ground wave till 500km (300 miles) and a similar ground and sky wave field strengths from 600 to 1100km (400 to 700 miles). Consequently, one much choose a transmitter at least 500km (300 miles) away, and preferably at more than 1100km (700 miles).
The upper limit for having a single hop is about 1900km (1200 miles) during the day (assuming a D-layer at an altitude of 70km) and 2100km (1300 miles) during the night (assuming an E-layer at an altitude of 90km).
Transmitters at greater distance will be received through two or more hops and will exhibit more than one sunrise and one sunset patterns.

You can found here a spectrum of all stations received at the SID monitoring station location (South of France). You will found, for instance, HWU on 18.3 kHz, GQD on 19.6 kHz, FTA on 20.9 kHz, DHO38 on 23.4 kHz.